Cable television distribution systems typically include a site called a head end where programming material is obtained and modulated onto appropriate carriers. The carriers are then combined for distribution to subscribers over what is typically referred to as the forward path. Signals coming from subscribers in a two way or “interactive” CATV system are typically routed in what is called the return path, and are received and routed to appropriate service providers. CATV signals are distributed and collected on a broadband network. CATV signals typically are transported from head end to subscriber and vice versa as RF or, in hybrid systems including optical transmission portions, RF is converted to light for fiber optic transmission over some portions of the signal path and as RF over other portions of the signal path. This process generally is bi-directional in the case of a hybrid two way CATV system. Depending on the particular architecture of the CATV plant there may exist intermediate signal processing locations.
The distribution for CATV is typically made up of conductors such as coaxial cable and optical fiber with periodic amplifiers to make up for signal loss due to a number of factors including, without limitation, imperfections in the conductors, splitting of the signal during distribution, and so on. In two-way systems, losses of the same general nature occur in the return path.
Of particular importance when transmitting signals in a CATV system is that the signals arrive at useful amplitudes. Among the other important performance criteria for a CATV system is that it not introduce excessive noise or produce distortion in the amplitude and/or phase of the signals. Methods currently exist for measuring these and other defects. For example, signal amplitude has traditionally been measured with a spectrum analyzer or calibrated tunable receiver called a signal level meter. Noise has sometimes been measured by temporarily removing a carrier and measuring the residual noise in the channel previously occupied by that carrier, or by making a noise measurement at an unoccupied frequency close to a frequency of interest.
Amplitude versus frequency distortion has sometimes been measured by inserting a moving, or swept, test carrier at one end of the system and observing it on (a) receiver(s) at various locations in the system, or by using carriers already on the system, possibly with some added carriers to provide a discrete series of frequency test measurements from which the continuous frequency response of the system can be inferred. Some television signals on CATV systems may also contain imbedded test signals called VITS (Vertical Interval Test Signals) which are added to standard TV signals during the vertical blanking interval. VITS contain bursts of several frequencies within the TV signal's bandwidth, which may be used for approximation of amplitude versus frequency response within the TV signal's occupied bandwidth.
Phase distortion within a TV signal's bandwidth can be measured using a so-called 2T pulse, which is a sine squared pulse located within the vertical interval. By demodulating the TV signal and observing the shape of the 2T pulse at various locations within the system, phase distortion can be approximated although the phase non-linearity versus frequency within the TV signal's bandwidth is not known.
At frequencies at which 2T pulses are not employed, or with non-TV signals, there is no simple method for measuring phase distortion. This application describes a convenient and simple-to-use system for measuring phase distortion in a CATV system.